rtl8187: adapt for deprecated IEEE80211_CONF_SHORT_SLOT_TIME flag

This updates 8187L handling for short slot after "mac80211: fix short
slot handling". For 8187B, there was no handling done for short slot
timing so nothing done, support for it will come in a following patch.

Signed-off-by: Herton Ronaldo Krzesinski <herton@mandriva.com.br>
Signed-off-by: John W. Linville <linville@tuxdriver.com>